Sodium hydroxide solution and chlorine gas are important base chemicals which are produced in large amounts. The modern methods for production of sodium hydroxide solution and chlorine are based on the electrolysis of aqueous sodium chloride solutions by the membrane method. The electrolysis procedure which is associated with high energy consumption proceeds at the membranes. These membranes are about 0.1 mm-thick, chlorine-resistant cation-exchanger membranes which consist of polytetrafluoroethene (PTFE/Teflon) having negatively charged SO3 substituents (Nafion). The anions formed in the electrolysis such as OH− or Cl− cannot pass through them, whereas the positively charged Na+ ions can pass through these membranes. Owing to the impermeability to Cl− ions, a 35% strength sodium hydroxide solution is formed which is virtually uncontaminated by sodium chloride.
The negatively charged SO3 groups of the membranes can absorb, in particular, divalent cations such as calcium, magnesium, strontium, barium and others. Precipitates occur on the membranes which subsequently then become blocked. As a result, the performance of the membranes decreases. The yield of the reaction of sodium chloride to give chlorine and sodium hydroxide solution decreases, falling current densities and increasing cell voltages are the consequence and, thereby, a higher specific energy consumption. At the same time, likewise, the service life of the membranes—until they must be replaced—decreases.
It is therefore absolutely necessary to provide a sodium chloride brine for the electrolysis procedure at the membranes, which brine is as free as possible of other cations except for sodium.
Producers of membranes give the following recommendations for purified sodium chloride brines for the residual concentrations of cations which the maximum which should be present in the brine after the precipitation and also fine purification with chelating resins (Bayer SP IOW 4006e, 2001-01).
Calcium 2+; magnesium 2+: <20 ppb
Strontium 2+: <100 ppb
Barium 2+: <500 ppb
The sodium chloride delivered by the manufacturers, when dissolved in water to give a sodium chloride brine, does not meet these high purity requirements.
The sodium chloride used as feed stock is a natural product which can contain, in addition to sodium chloride, a further up to 85 chemical elements. In addition, the content of sodium chloride varies considerably in the natural product sodium chloride.
The sodium chloride delivered which is used for obtaining chlorine and sodium hydroxide solution in electrolysis methods, should have a content of at least approximately 97% by weight of NaCl.
Depending on origin, sodium chloride additionally contains differing amounts of divalent and higher valent cations such as the abovementioned cations and also, in traces, zinc, cobalt, iron, nickel, chromium, copper, potassium inter alia, and also various anions.
For example, an analysis of the most important minerals of the natural rock salt of the Zechstein age which were formed approximately 280 million years ago by drying out of the sea, carried out by Geo-Anal-Speck in Krakow in 1991 using the spectrophotometric method (AAS) showed a content of 0.271 ppm of Ca, 0.03 ppm of Mg, 4.1 ppm of Zn, 142 ppm of Fe, 2.7 ppm of Co, 432 ppm of Ni, 0.34 ppm of Cu, 0.09 ppm of Cr and also 6.8 ppm of Mn, based on 100 g of salt. Independently of the alkaline earth metals, said heavy metals can lead to damage to the electrodes used during the electrolysis procedure, preferably the gas diffusion electrodes used today.
The concentration of sodium chloride in the aqueous sodium chloride solution which is used for the electrolysis is in the range of 250-330 grammes of NaCl per liter of solution.
In particular, the concentration of divalent cations should be removed for the above-mentioned reasons before the electrolysis procedure as quantitatively as possible, since they adversely affect the electrolysis procedure.
Therefore, even now an aqueous sodium chloride solution to be used in alkali metal chloride electrolysis is purified in two steps before the electrolysis; in a first step, by precipitation, a relatively large amount of the divalent cations are removed. Their concentration in the aqueous brines after precipitation is then in the range from approximately 0.1 to approximately 20 ppm. In a second step, the concentration, in particular of the divalent cations calcium, magnesium, strontium, barium in the brine is reduced by overfiltration over chelating exchangers to residual values in the aqueous effluent out of the column, depending on cation type, of below approximately 500 ppb to approximately 20 ppb.
If the concentration of divalent cations, individually or in sum, exceeds the said values in the column eluate, the column is regenerated.
The selectivity of chelating ion exchangers for removing the divalent cations calcium, magnesium, strontium, barium from sodium chloride brines differs. The following sequence results in decreasing selectivity: magnesium>calcium>>strontium>barium>alkali metals (Na). Barium ions are therefore absorbed markedly more poorly than magnesium ions and calcium ions.
The producers of the membranes take this into account in such a manner that they recommend the abovementioned differing residual values in the purified brine for the divalent cations calcium, magnesium, strontium, barium.
The purpose must therefore be to develop a chelating resin having improved selectivity which, in addition to a generally high absorption capacity for divalent cations, in particular displays a high absorption capacity for cations such as barium.
The amount of divalent cations which the chelating resin has absorbed until it exceeds the concentration greater than the limiting value which, for calcium and magnesium, is 20 ppb of cations in the aqueous effluent of the chelating resin, is in this case called the dynamic absorption capacity of the chelating resin.
The brine thus purified is converted electrolytically to sodium hydroxide solution and chlorine—see R. M. Klipper, H. Hoffmann, T. Augustin, Ion Exchange Advances, Proceedings of IEX 92, pages 414 ff.; Elsevier Applied Science, London, 1992.
If the chelating ion exchanger is loaded with the abovementioned cations such as calcium, magnesium, strontium, barium, to the extent that the cations can no longer be sufficiently removed and in the eluate values of calcium and magnesium >20 ppb are measured, the chelating ion exchanger must be regenerated with acids, preferably hydrochloric acid. The chelating groups are converted to the hydrogen form. Then, the chelating resin is converted by means of aqueous sodium hydroxide solution into the sodium form of the chelating groups and can then again remove divalent cations from the brine.
Chelating ion exchangers having a high dynamic absorption capacity for divalent cations such as calcium, magnesium, strontium, barium from aqueous salt brines, in particular sodium chloride brines, for low residual values of cations in the purified solution, a high degree of utilization of the functional groups during loading, and also high regeneration efficiency with simultaneously high bead stability of the chelating resins, are sought.
In U.S. Pat. No. 4,818,773, chelating resins having iminodiacetic acid groups are described and their absorption capacity for cations such as calcium and magnesium from sodium chloride brines is tested in the brine test.
The dynamic absorption capacity thereof for calcium ions was determined. In the case of Amberlite® XE 318, a chelating resin with iminodiacetic acid groups, it is 7.1 grammes of calcium per liter of resin. 520 bed volumes of brine were filtered through the chelating resin in 26 hours—at a rate of 20 bed volumes (BV) per hour.
The concentration of calcium in the purified brine is 20 ppb, the loading of the chelating resin with sodium chloride brine was ended when the calcium concentration is >500 ppb in the effluent of the chelating resins.
DE 3 704 307 A1 describes chelating resins having iminodiacetic acid groups and their dynamic absorption capacity for cations such as calcium and magnesium from sodium chloride brines was tested in the brine test.
The dynamic absorption capacity thereof for calcium ions was determined. For Lewatit OC 1048 from Bayer AG, a macroporous chelating resin having iminodiacetic acid groups, it is 4.2 grammes of calcium per liter of resin, Example 2.
The calcium concentration in the purified brine is for the majority of the loading cycle <20 ppb, and the loading of the chelating resin with sodium chloride brine is terminated when the calcium concentration is >50 ppb in the effluent of the chelating resins.
EP 1 078 690 A2 discloses a method for producing monodisperse ion exchangers having chelating groups, wherein, as chelating groups, (CH2)n—NR1R2 are present in the ion exchanger and R1 is inter alia H, CH2—COOH and R2 is inter alia CH2—COOH and n is an integer between 1 and 4.
The chelating resins described in the prior art do not meet the requirements for fine purification of salt brines in the currently used membrane electrolysis methods. The residual amount of calcium in the purified brine in U.S. Pat. No. 4,818,773 is 20 ppb. The residual amount of calcium in the purified brine in DE 3 704 307 A1 is only at times <20 ppb. In addition, the amount of absorbed calcium is not sufficiently high, and so in U.S. Pat. No. 4,818,773 the loading is terminated as soon as after 26 hours and then regeneration must proceed. In long-term operation, the chelating resin must therefore be regenerated several hundred times per year. The chelating resins of EP 1 078 690 A2, in particular with the simultaneous presence of calcium and barium, do not display the barium selectivity required for current-day membrane processes.
The object of the present invention is therefore the provision of novel chelating exchangers for markedly better removal                of alkaline earth metals such as magnesium, calcium, strontium and in particular barium in the simultaneous presence of calcium, from aqueous brines, as customarily occur in chloralkali electrolysis,        of heavy metals and noble metals from aqueous solutions or organic liquids or vapours thereof, particularly of mercury from aqueous solutions of alkaline earth metals and alkali metals, in particular a removal of mercury from brines or alkali metal chloride electrolysis.        